As is known, a piezoelectric thin film resonant element has a structure in which a piezoelectric film is sandwiched by electrodes on its upper surface and a lower surface, with a cavity or an acoustic multi-layer film provided below the lower electrode. The piezoelectric thin film resonant element converts an electric signal, applied across the upper and lower electrodes to the piezoelectric film, into a mechanical displacement of the piezoelectric film. In addition, the piezoelectric thin film resonant element responds to a mechanical displacement of a specific frequency, depending on the total thickness of the piezoelectric film, the upper electrode and the lower electrode, and also on the cavity below the lower electrode or the acoustic multi-layer film, and then re-converts the mechanical displacement into an electric signal to be outputted outside.
A piezoelectric thin film resonant element provided with a cavity below the lower electrode is called a “FBAR (Film Bulk Acoustic Resonator)” type. A piezoelectric thin film resonant element provided with an acoustic multi-layer film below the lower electrode is called a “SMR (Solidly Mounted Resonator)” type.
An FBAR type piezoelectric thin film resonant element has a basic structure as illustrated in FIG. 22 and FIG. 23, for example. FIG. 22 is a plan view of an FBAR piezoelectric thin film resonant element, and FIG. 23 is a sectional view taken along lines X-X in FIG. 22. The dotted area in FIG. 22 indicates a resonant portion of the piezoelectric thin film resonant element.
A piezoelectric thin film resonant element 100 includes, as primary constituent members, a substrate 101, a lower electrode 102, a piezoelectric film 103, an upper electrode 104, a terminal electrode 102A and a terminal electrode 104A. The lower electrode 102, the piezoelectric film 103 and the upper electrode 104 are rectangular and are laminated on an upper surface of the substrate 101 in this order. The lower electrode 102 and the upper electrode 104 has the substantially same area, and the piezoelectric film 103 has a larger area than that of the lower electrode 102 or the upper electrode 104. The substrate 101 is formed, correspondingly to where the lower electrode 102 and the upper electrode 104 face each other, with a cavity 105 whose opening is slightly larger in area than the mutually facing portion of the two electrodes.
When a high-frequency signal is applied between the upper electrode 104 and the lower electrode 102, an elastic wave is excited in the piezoelectric film 103 due to the inverse piezoelectric effect. The elastic wave includes a wave 106b (hereinafter called “vertically vibrating wave”) which vibrates along a plane in the thickness direction (zy plane in FIG. 23) of the piezoelectric film 103, and a wave 106a (hereinafter called “horizontally vibrating wave”) which vibrates along a plane which is parallel with the film face (xy plane in FIG. 22) of the piezoelectric film 103.
The vertically vibrating wave 106b is reflected by an end surface of the piezoelectric film 103 on the upper electrode 104 side and by an end surface of the film on the lower electrode 102 side. The reflection induces a resonance inside the piezoelectric film 103, with the vertical elastic wave 106b of a prescribed frequency determined by two factors, i.e. a total thickness H of the piezoelectric film 103, the upper electrode 104 and the lower electrode 102, and a propagation velocity V of the elastic wave which is determined by the materials used in these parts. The vertical elastic wave 106b of the other frequencies is attenuated. Thus, the vertical elastic wave 106b includes a frequency (resonant frequency) f which satisfies the relationship f=n×V/2H (where n indicates an integer), and the vertical elastic wave 106b of this resonant frequency is re-converted into an electric signal and outputted.
As described above, the piezoelectric thin film resonant element 100 has a system for converting a high-frequency electric signal (electric energy) into an elastic wave (mechanical energy) of a specific frequency by using the inverse piezoelectric effect and the resonance phenomenon based on a mechanical structure, and a system for re-converting the elastic wave (mechanical energy) of the above-described specific frequency into an electric signal (electric energy) of this specific frequency. Of the elastic waves which are generated in the process of electrical-mechanical energy conversion, mechanical energy of the horizontal elastic wave 106a is not very well re-converted into the electric energy, and therefore the horizontal elastic wave 106a causes energy loss in the energy conversion process in the piezoelectric thin film resonant element.
Also, the horizontal elastic wave 106a causes spurious modulation affecting the resonance characteristics of the piezoelectric thin film resonant element 100 and deteriorates amplitude characteristic and phase characteristic of the resonance characteristics. As a result, when a plurality of the piezoelectric thin film resonant elements 100 are combined to provide a filter, ripples appear over the pass band of the filter, and in addition it is likely that the filter's insertion loss, group delay characteristic, etc. are decreased.
Piezoelectric thin film resonant elements are devices which can be utilized suitably in filters for extraction of a specific frequency from high-frequency electric signals. For example, they feature lower loss than those resonant elements which utilize SAW (Surface Acoustic Wave), and they are superior in electric power characteristic, ESD (electro-static discharge) characteristic, etc. For these advantages, there is an increasing demand for use as a constituent part in transmission/reception filters, branching filters, etc. for portable wireless equipment.
Transmission filters and branching filters used in portable wireless equipment or the like are required to achieve low power consumption. Reception filters are required to have a high reception sensitivity requirement. Therefore, the piezoelectric thin film elements are required to achieve a decreased energy loss and to have a high Q value.
With the above-described situation, various methods have been proposed so far, such as a method for increasing the Q value of the piezoelectric thin film resonant element, a method for decreasing energy loss and spurious modulation by suppressing the horizontal elastic wave.
For example, Japanese National Publication No. 2003-505906 (Patent Document 1) and Japanese Lain-open Patent Publication No. 2006-5924 (Patent Document 2) disclose a technique for reducing spurious modulation by making a piezoelectric film 103 of a material which has a dispersion relationship k(ω) where the wave number k takes a real number in a range where the angular frequency ω is lower than the cutoff frequency ωc, and by configuring an outer edge film thickness of the upper electrode 104 smaller than the film thickness of the inner side portion as illustrated in FIG. 24. Note that FIG. 24 is similar to but is different from FIG. 23 in that the outer edge of the upper electrode 104 is provided with a step 104 a, whose film thickness H2 is smaller than the film thickness H1 of the inner portion. The piezoelectric film 103 which has the above-defined dispersion relationship k(ω) can be provided by with a homogeneous material which has a Poisson's ratio not greater than ⅓, and provided by e.g. aluminum nitride (AlN).
Also, Japanese Lain-open Patent Publication No. 2006-109472 (Patent Document 3) discloses a technique for increasing the Q value of resonance characteristics by making a piezoelectric film 103 of a material which has the above-described dispersion relationship k(ω), and by configuring an outer edge film thickness of the upper electrode 104 larger than the film thickness of the inner side portion as illustrated in FIG. 25. Note that FIG. 25 is similar to but is different from FIG. 23 in that the outer edge of the upper electrode 104 is provided with a projection 104b, whose film thickness H3 is larger than the film thickness H1 of the inner portion.
Meanwhile, Japanese Lain-open Patent Publication No. 2006-128993 (Patent Document 4) discloses a technique for inhibiting leakage of the horizontal elastic wave 106a by removing portions of a piezoelectric film 103 which protrude from the upper electrode 104 as illustrated in FIG. 26. FIG. 26 illustrates the piezoelectric film 103 as having a width (dimension in the y direction) slightly larger than the width of the upper electrode 104. However, this is a drawing technique to clarify that the piezoelectric film 103 exists. The actual width of the piezoelectric film 103 is substantially the same as that of the upper electrode 104.
Japanese Lain-open Patent Publication No. 2006-128993 (Patent Document 4) also discloses a technique for inhibiting leakage of the horizontal elastic wave 106a and thereby improving the Q value of resonance characteristics and an electromechanical coupling coefficient. As illustrated in FIG. 27, an additional electrode 107 is provided in this technique on an upper surface of a terminal electrode 104A of an upper electrode 104. In FIG. 27, a distance D is ensured between a tip of the additional electrode 107 and a tip of the lower electrode 102. This is based on a fact that an overlap made by the tip of the additional electrode 107 and the tip of the lower electrode 102 will result in deterioration of the characteristics. That is, the distance D is ensured to prevent the tips from overlapping in the manufacturing process.
Patent Document 1: Japanese National Publication No. 2003-505906
Patent Document 2: Japanese Lain-open Patent Publication No. 2006-5924
Patent Document 3: Japanese Lain-open Patent Publication No. 2006-109472
Patent Document 4: Japanese Lain-open Patent Publication No. 2006-128993
Japanese National Publication No. 2003-505906, Japanese Lain-open Patent Publication No. 2006-5924 and Japanese Lain-open Patent Publication No. 2006-109472 disclose techniques for decreasing spurious modulation and improving the Q value of resonance characteristics by adjusting the film thickness of the outer edge of the upper electrode 104 of the piezoelectric thin film resonant element 100. In these techniques, however, increasing the film thickness tends to improve the spurious modulation but decrease the Q value of the resonance characteristics, while decreasing the film thickness tends to improve the Q value of the resonance characteristics but aggravate the spurious modulation.
This tendency is understood by referencing FIG. 28. FIG. 28 defines regions and their impedances in the structure in FIG. 25. Specifically, the laminated structure composed of the lower electrode 102, the piezoelectric film 103 and the upper electrode 104 is divided into three regions: Region (B) in which the projection 104b provided at the outer edge of the upper electrode 104 overlaps the lower electrode 102, Region (C) which is outside of Region (B), and Region (A) which is inside of Region (B). Further, acoustic impedances of Regions (A), (B) and (C) are defined as ZA, ZB and ZC, respectively.
If the projections 104b are provided at the outer edges of the upper electrode 104, since the thickness HA, HB, HC of Regions (A), (B), (C) are in the relationship HC<HA<HB, the acoustic impedances ZA, ZB, ZC are in the relationship ZC<ZA<ZB. Since the acoustic impedance ZB of Region (B) is larger than the acoustic impedances ZA, ZC of Regions (A), (C), an acoustic impedance mismatch between Region (A) and Region (C) is increased. Hence, higher-order, symmetric and asymmetric elastic waves of the horizontal mode are reflected by Region (B), and become less likely to leak to Region (C).
As a result, a decrease in the leakage of the horizontal elastic waves near the primary vibration frequency causes improvement of the Q value of resonance characteristics of the piezoelectric thin film resonant element 100. In Region (A), however, due to the reflection of the horizontal elastic waves in Region (B), standing wave of the horizontal elastic wave, which can cause spurious modulation, is likely to be generated. Therefore, the spurious modulation in the piezoelectric thin film resonant element 100 is aggravated.
On the other hand, as illustrated in FIG. 24, if a step is provided at the outer edge of the upper electrode 104, since the thickness HA, HB, HC of Regions (A), (B), (C) are in the relationship HC<HB<HA, the acoustic impedances ZA, ZB, ZC are the relationship ZC<ZB<ZA. Since the acoustic impedance ZB of Region (B) is between the acoustic impedances ZA, ZC of Regions (A), (C), respectively, an acoustic impedance mismatch between Region (A) and Region (C) is decreased. Hence, higher-order, symmetric and asymmetric elastic waves of the horizontal mode are more likely to leak to Region (C).
As a result, due to the leakage of horizontal elastic waves from Region (B) to Region (C), the standing wave of the horizontal elastic standing waves, a cause of the spurious modulation, is unlikely to be generated in Region (A). Therefore, the spurious characteristics of the piezoelectric thin film resonant element 100 is improved. However, at the same time, since leakage of the horizontal elastic waves near the main vibration frequency is increased, the Q value of the resonance characteristics of the piezoelectric thin film resonant element 100 is deteriorated.
Hence, simple adjustment such as making the thickness of Region (B) greater than those of Regions (A), (C) or to be between the thickness of Region (A) and that of Region (C) does not achieve improvement of both of the Q value of resonance characteristics and the spurious characteristics of the piezoelectric thin film resonant element 100.
Next, the method, which is disclosed in Japanese Lain-open Patent Publication No. 2006-128993, and in which an additional electrode 107 is provided on an upper surface of the terminal electrode 104A of the upper electrode 104 has the following problem:
Regarding manufacturing a piezoelectric thin film resonant element 100, if the edge of the lower electrode 102a is right-angled, a crack is likely to be formed at the edge portion and the membrane serving forming the resonant portion is likely to be destroyed, whereby reliability is deteriorated. Therefore, an inclined surface 102a is provided at a tip of the lower electrode 102 as illustrated in FIG. 29. Accordingly, an incline is formed in the piezoelectric film 103 formed on the lower electrode 102, and an inclined surface 104c is likely to be formed in the upper electrode 104 formed on the piezoelectric film 103. As a result, the inclined portions in the lower electrode 102, the piezoelectric film 103 and the upper electrode 104 are thinner than the other portions, and the acoustic impedance changes in the inclined portions.
Specifically, as illustrated in FIG. 29, laminated portions constituted of the lower electrode 102, the piezoelectric film 103 and the upper electrode 104 are divided into three regions; i.e. Region (B) in which the slanted surface 102a of the lower electrode 102 faces the slanted surface 104c of the upper electrode 104, Region (C) which is outside the Region (B), and Region (A) which is inside Region (B). The acoustic impedances ZA, ZB and ZC of Regions (A), (B) and (C), respectively, are in relationship ZC<ZB<ZA.
This magnitude relation between the acoustic impedances is the same as in the case illustrated in FIG. 24 in which a step 104a is provided on the outer edge of the upper electrode 104. The method in which an additional electrode 107 is provided on the upper surface of the terminal electrode 104A of the upper electrode 104 disadvantageously includes a factor which deteriorates the Q value of resonance characteristics in the piezoelectric thin film resonant element 100.